Photoplotters are widely used to generate precision artwork for the creation of printed circuit boards. Two primary techniques are used in the design of current photoplotters. Calligraphic plotters draw with light on film much like pen plotters are used to draw with a pen on paper. Commonly, light from a quartz halogen lamp or a xenon flash tube is passed through one of a selectable set of apertures, focused on the film and moved by some sort of two axis orthogonal mechanism. Raster photoplotters employ a scanning technique similar to the way images are created by an electron beam on a cathode ray tube. With raster photoplotters, a laser light source is commonly focused onto a highspeed acousto-optic modulator which turns the beam on and off. The modulated beam is then imaged onto a rapidly rotating polygonal mirror which scans the beam across the face of a light sensitive film. The film moves perpendicularly to the modulated beam. The plot is imaged line by line as a series of dots.
The presently used techniques have advantages and disadvantages. Raster photoplotters are relatively quick (30 seconds to 3 minutes per page). The plot time is independent of image density. However, the costs to build, test and service these raster photoplotters make them quite expensive. Calligraphic plotters, on the other hand, exhibit plot times directly proportional to the density of the plotted image. For a densely imaged plot, plot times can exceed several hours. However, calligraphic plotters, although not inexpensive, are substantially cheaper than raster plotters.